Gold plating of metal surfaces of electronic devices is often essential for providing reliable, low resistance electrical contact with the metal surfaces. This is particularly true of metal surfaces made of materials that naturally form an oxide passivation layer. Such materials include, for example, stainless steels.
Stainless steel is “stainless” because it forms a generally stable chromium oxide which is impervious to most chemicals. This resistance to chemical attack also makes stainless steel a challenging surface for electroplating gold and achieving good adhesion of the plated gold to the stainless steel surface.
Typically, electroplating of gold to stainless steel uses an acid/chloride solution to plate a relatively thin nickel “strike” layer onto the stainless steel. Gold is then electroplated over the nickel layer, which may also be known as a “tie” layer. This works well, so long as the nickel is completely encapsulated by the gold. However, should any nickel be exposed, for example at an edge of a photoresist defined gold/nickel pattern, then a galvanic reaction will occur when the metals come into contact with conductive solutions in subsequent processing steps, such as commonly used metal cleaning processes. The galvanic reaction corrodes the nickel layer and undercuts the gold layer. Undercutting the gold layer destroys the integrity of the patterned gold/nickel structure.
Thus, for applications requiring a patterned gold structure, it is desirable to plate the gold directly onto the stainless steel surface. What is needed is a photoresist compatible gold plating process that results in good adhesion between the gold layer and the stainless steel surface without introducing a “tie” layers susceptible to corrosion or galvanic dissolution.
Gold (I) cyanide chemistry has also been used for electroplating gold. However, gold(I) cyanide does not perform well at a low pH condition typically used for electroplating solutions for stainless steels. For example, below a pH of 4, the gold (I) cyanide complex starts to disassociate (disproportionation), such that the gold begins to precipitate and the cyanide may be released as a toxic gas. Some forms of gold (III) chloride, such as hydrogen gold (III) tetrachloride (HAuCl4), may be stable below a pH of 4. However, gold (III) chloride plating solutions do not produce an electrodeposited gold layer with good adhesion to stainless steel.